1. Field of the Invention
The present invention concerns a hard disk drive (HDD), and more particularly a method of writing the servo pattern for detecting fine defects in the servo burst signals and a drive circuit therefor.
2. Description of the Related Art
HDDs are widely used as auxiliary memory devices of computer systems because they provide means for accessing a large amount of data at high speed. In addition, many efforts have been made to increase the data storage capacity of an HDD in order to meet the needs in the informational age, i.e., to increase the number of the tracks per inch (TPI) of the platters (disks). High TPI density involves problems relating to servo control because the narrowed gap between the tracks reduces the off-track margins of the position error signals (PES) by which the heads follow the tracks. The servo pattern of a conventional HDD is described with reference to FIG. 1 illustrating the conventional format of the servo sector.
The servo sector consists of a preamble portion, servo address mark (SAM) portion, gray code portion, portion of the burst signals A, B, C, D, and timing margin portion PAD. The preamble provides a servo sync signal for reading the servo data together with a gap to indicate of the servo sector. The SAM indicates the start of the servo providing a sync signal to read the following gray code. The gray code 10, 11, 12, 13 provides the identification data ID of each track. The track data is changed by one bit when the heads move through the gray codes, written by half track in the servo writer. The burst signals A, B, C, D provide the position error signals PES required to follow the tracks. In the embedded sector servo mechanism, the burst signals C, D are usually employed for positioning the heads on-track, the burst signal A for distinguishing the even numbered tracks, and the burst signal B for distinguishing the odd numbered tracks. The PAD provides a timing margin required after reading the servo data.
Referring to FIG. 2 illustrating the on-track timing diagrams of the burst signals A, B, C, D read from the disk prepared with a servo sector format as shown in FIG. 3, reference symbol 2a indicates the servo sector of a certain track, signal BS the burst signals A, B, C, D read by a head and amplified, signal SE a sample & hold enable signal, signal FBS the full wave rectified burst signals A, B, C, D obtained through an automatic gain control (AGC) circuit, and signals SA to SD the burst signals A, B, C, D applied to sample & hold circuits. Signal RS is a reset signal to discharge the burst signals A, B, C, D applied to the sample & hold circuits. Among these signals SA to SD, SA has the highest level and SB the lowest level while SC and SD have the same level which is midway between SA and SB. The charged level of each of the signals SA to SD corresponds to the area of the eight half waves of the corresponding signal. The signals SA to SD are maintained at the charged levels until the reset signal RS is enabled.
In such burst signal detection of sample & hold, the charged levels using the sampled burst signals vary with the pulse number and area of the burst signals. Namely, a disk having a high track density burst to have a high storage capacity has defects of various sizes, which, if they exist in the portion of the burst signals A, B, C, D after the servo writing, considerably affect the position error signals PES. The conventional HDD may not detect the fine defects existing in the burst signals by employing the sampled signals except that the defects are large enough to influence the levels of the sampled signals, thereby resulting in errors in the servo control.